Field of the Invention
The present invention relates to a diffusing agent composition, a method of forming an impurity diffusion layer, and a solar cell.
Description of the Related Art
According to the related art, in manufacturing a solar cell, a P-type or N-type impurity diffusion layer is formed in a semiconductor substrate by forming a coating film on a surface of the semiconductor substrate using a diffusing agent that contains an impurity diffusion component and then by causing the impurity diffusion component to diffuse from the coating film of the diffusing agent into the semiconductor substrate.
In manufacturing a solar cell, a spin coating method is often used as a method for applying a diffusing agent on a surface of a semiconductor substrate. However, efforts have been also made toward employment of a screen printing method and a roll coat printing method. In the screen printing method, a screen (printing plate) of mesh-like silk, synthetic resins, stainless steel, or the like is first stretched on a frame, and a part that allows a diffusing agent to pass and a part that does not allow a diffusing agent to pass are formed on the screen. The diffusing agent is then applied to the screen, and the applied diffusing agent is pushed out onto a surface of a semiconductor substrate using a squeegee. With this, the diffusing agent is transferred onto the surface of the semiconductor substrate, thereby forming a coating film of the diffusing agent in a predetermined pattern, line, or the like on the surface of the semiconductor substrate.
In the roll coat printing method, a printing roller (printing plate) on which a groove is formed along the circumference thereof and a pressing roller for pressing a semiconductor substrate against the printing roller are arranged facing each other at a short distance. Then, the printing roller and the pressing roller are rotated in directions opposite from each other while feeding the diffusing agent into the groove, and the semiconductor substrate is allowed to pass between these rollers. With this, the printing roller and the semiconductor substrate come into contact with each other with pressure, and the diffusing agent filling the groove of the printing roller is transferred onto the surface of the semiconductor substrate, thereby forming a coating film of the diffusing agent in a predetermined pattern, line, or the like on the surface of the semiconductor substrate.
For example, Patent Document No. 1 describes a dopant paste (diffusing agent composition) for use in these printing methods.
[Patent Document No. 1] JP 2002-539615 (published Japanese translation of PCT international publication for patent application)
As described above, efforts have been currently made toward employment of a screen printing method and a roll coat printing method in manufacturing a solar cell. However, it is difficult to form a coating film, using these methods, at a level that ensures practical use. One of the causes for this is that diffusing agents that can be preferably employed for these methods are not known. In other words, since a diffusing agent is applied to a mesh-like or roll-like printing plate in the above-stated screen printing method and roll coat printing method, the diffusing agent needs to have a predetermined viscosity. In order to provide a viscosity, the solid content concentration of a conventional diffusing agent is set to be high to some extent. However, this makes the diffusing agent easy to dry. When a diffusing agent applied to a printing plate is dried, print cracking is formed on a semiconductor substrate, preventing the formation of a favorable coating film.
Therefore, diffusing agents used for the screen printing method and roll coat printing method are required to be hard to dry while having a predetermined viscosity. Meanwhile, for diffusing agents, there is always a demand for the improvement of the ability to create a precise coating film form (pattern) when applied on the surface of a semiconductor, i.e., coating film formability, and the ability to uniformly diffuse over a predetermined region of the semiconductor substrate so as to reduce a resistance value at a diffusion region to a desired value, i.e., diffusibility.
The present invention provides a diffusing agent composition that can be preferably employed for a screen printing method and a roll coat printing method having excellent coating film formability and diffusibility; a method of forming an impurity diffusion layer using the diffusing agent composition; and a solar cell.
A diffusing agent composition according to one embodiment of the present invention that is used to print an impurity diffusion component onto a semiconductor substrate, comprises: an impurity diffusion component (A); a binder resin (B) configured to thermally decompose and disappear below a temperature, at which the impurity-diffusing component (A) starts diffusing thermally; a SiO2 fine particle (C); and an organic solvent (D) configured to contain an organic solvent (D1) having a boiling point of at least 100 degrees Celsius.
According to this embodiment, a diffusing agent composition can be obtained that can be preferably employed for a screen printing method and a roll coat printing method having excellent coating film formability and diffusibility.
Another embodiment of the present invention relates to a method for forming an impurity diffusion layer, and the method for forming an impurity diffusion layer comprises: forming a coating film by printing the diffusing agent composition according to the above embodiment; and diffusing the impurity diffusion component (A) of the diffusing agent composition into a semiconductor substrate.
According to the embodiment, an impurity diffusion layer can be formed with higher accuracy.
Yet another embodiment of the present invention relates to a solar cell, and the solar cell comprises a semiconductor substrate in which an impurity diffusion layer is formed by the method of forming an impurity diffusion layer of the embodiment.
According to the embodiment, a more reliable solar cell can be obtained.
Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: